1. Field of the Invention
The invention concerns coloured oxidised aluminium pigments, a process for the production thereof and use thereof.
2. Description of the Prior Art
Aluminium pigments are used widely in coatings as special-effect pigments. The term special-effect pigments is used to denote pigments which have a directed reflection at oriented, metallic or highly light-refractive particles of a predominantly flat configuration (German Standard DIN 5594). They are always of a plate-like or flake-like configuration and have very large particle diameters compared with dye pigments. Their optical properties are determined by reflection and interference. Depending on transparency, absorption, thickness, single-layer or multi-layer structure, the special-effect pigments exhibit a metallic shine, a pearl shine, interference or interference reflection. The main area of use is in cosmetics and the automobile sector, and in addition in colouring plastic materials, paints, leather coatings, the printing industry and the ceramic industry. (For a comprehensive representation of the technical background, see W. Ostertag, Nachr. Chem. Tech. Lab. 1994, 9, 849).
The aluminium pigments which are most frequently used are aluminium flakes or pigments based on flake-like Cu/Zn-alloys and coated mica flakes, wherein aluminium pigments exhibit a typical metal shine whereas coated mica flakes exhibit a typical pearl shine.
In recent years the need for coloured special-effect pigments has increased greatly. Therefore for example oxide-covered copper and brass flakes, substrates which are coated with transition metal oxides such as muscovite, phlogopite or glass, guanine single-crystals (fish silver), BiOCl-single crystals, flake-form haematite single-crystals, flake-form phthalocyanines, micronised titanium dioxide, polished aluminium shot, iron oxide or crushed thin multi-layer films with a Fabry-Perot-structure were used as special-effect pigments.
In comparison, by colouring aluminium pigments, it is possible to produce coloured pigments with improved covering capability, compared with pearl shine pigments, and good colouristic options. In that respect, the colouring action is produced either by fixing colour pigments by means of polymers, by coating with oxides of different metals using a very wide range of different processes, by coating with a colour pigment-bearing oxide layer or by oxidation.
In accordance with U.S. Pat. No. 4,328,042 and EP-A-0 033 457 aluminium flakes are coloured by the deposition of iron oxide from iron pentacarbonyl, using a technically very expensive fluidised bed process. That procedure gives rise to gold-coloured aluminium pigments.
In accordance with U.S. Pat. No. 5,037,475 colour pigments are fixed on the metal surface by carboxyl group-bearing polymers. The pigments obtained however have only a low level of colour intensity.
Aluminium pigments are coloured in accordance with WO 91/04293 (PCT/US90/05236) by the fixing of polymer-coated colour pigments on the metal surface by means of electrostatic forces.
In accordance with EP-A-0 238 906 metal pigments are covered with a titanium dioxide layer by the controlled hydrolysis of an organic titanate ester compound. Various colour shades can be achieved by varying the thickness of the oxide layer. For that purpose it is necessary to observe accurately controlled reaction conditions such as pH-value and the rate of adding material by dropping. In order to achieve colour effects, it is also necessary to perform a calcination operation which however can only be carried out with difficulty, because of the low melting point of aluminium.
U.S. Pat. No. 4,978,394 describes the production of titanium dioxide-coated aluminium pigments by chemical vapour deposition (CVD) which is technically highly expensive.
U.S. Pat. No. 4,158,074 discloses the production of coloured aluminium pigments by coating with a film of hydrated metal oxide. The film is produced by the treatment of fine aluminium flakes or plate portions in an alkaline solution of an iron, nickel, cobalt, zinc or copper salt at elevated temperature by electrochemical reaction of the metal salts.
U.S. Pat. No. 5,261,955 discloses a sol-gel process for the production of coloured metal pigments, wherein the metal flakes are dispersed in a sol of an inorganic salt, dispersed after filtration in a solution of an inorganic compound, for example cobalt nitrate, in an organic solvent and finally a sol-gel layer is formed on the flakes by heating.
In accordance with DE 1 95 01 307.7 (Eckart-Werke) aluminium pigments can be coloured in a very wide range of different colour shades such as for example blue, red, violet and gold, in accordance with a process which is simple from the point of view of the apparatus used, by the controlled hydrolysis of metal acid esters in the presence of colour pigments in an organic solvent.
JP-A-61-130375 discloses a gold-coloured aluminium pigment, produced by the treatment of aluminium powder with dichromate, sodium fluoride and surface-active agents in acid solution, drying and treatment with a fatty acid derivative. Colour shades other than gold cannot be achieved with that process. In addition the toxicity of the chemicals used and their high price represent a major disadvantage of the process.
U.S. Pat. No. 3,067,052 describes coloured aluminium pigments which are produced by the oxidation of aluminium powder with KMnO.sub.4 -solution, possibly with the addition of a reducing agent. The colour shade of these pigments is golden, possibly also with a greenish or reddish shade, depending on the respective reducing agent used. In this case also the toxicity of the oxidising agent has a detrimental effect.
The known processes for the colouring of aluminium pigments are all complicated, expensive or disadvantageous because of the toxicity of the reagents used.